Optical disks are becoming more and more prevalent for the use of recording information. One form of optical recording disks is called a CD-R or a recordable compact disk. The Photo CD is an example of this CD-R media. Typically, this type of disk has a transparent substrate, a recording layer formed on a substrate, and a reflective layer on the recording layer. The recording layer is essentially a photo absorption material made of mixture of some organic dye materials and is formed by spin coating. The recording materials used for CD-R applications have been described in U.S. Pat. Nos. 4,940,618; 5,604,004; 5,294,471; European Patent Application 0353393; and Canadian Patent 2,005,520. Commercial useful materials of the type described in these references have stringent requirements. One of these requirement is light stability. Since the Photo CD is a consumer product, it must be capable of withstanding extreme environment. The stability of the disk mainly depends on the nature of the recording layer and the reflector layer and their mutual interaction; and the protective overcoat. The above applications disclose phthalocyanine dye, metallized formazan dye and cyanine dye having excellent light stability. The reflecting layer is usually selected to be gold or a gold alloy because of its nobleness and high reflectivity. The CD-R specifications require that it has a high reflectivity of more than 60% similar to the compact disks.
During recording, writing laser light passes through the plastic substrate and is focused on the dye recording layer which is heated to change the dye material to form a mark. In other recording materials, recording pits are formed. The reflectivity of a mark is lower than in those areas not having the mark. Marks are formed as a change in the optical properties of the recording layer. In any event, the combination of some or all of these changes forms marks which can then be read back by the focused read laser beam. The record thus consists of marks of relatively low reflectivity on a background of relatively high reflectivity in relation to the read beam.
Thin layer of gold is normally used as a main reflection material in the reflective layer. It is a noble metal with a very high stability and does not introduce problems into the recording stability. When other metals which have a high reflectivity such as aluminum, silver and copper are used instead of gold, they have a problem in that they are reactive and can form oxides or other corrosive layers. The recording stability of these types of disks varies over time and degrades. However, materials such as silver are much less expensive than gold, and it would be highly desirable to use them. In fact, silver has about 5-7% higher reflectivity than gold.